Some weight-loss supplements labeled as OxyElite Pro Super Thermogenic contain the prescription drug fluoxetine, better known as Prozac.
In a public notification letter released Saturday, the Food and Drug Administration (FDA) warned consumers "not to purchase or use" the product.
But the supplement's manufacturer, Texas-based USPlabs, said that the tainted product is a counterfeit, most likely marketed by a Brazilian website.
"Neither USPlabs nor its contract manufacturers has ever ordered or in any way handled fluoxetine for any purpose of any kind," a company statement said.
The FDA found fluoxetine in bottles labeled OxyElite Pro Super Thermogenic during an "examination of international mail shipments," the FDA letter said.
The product examined by the agency had an expiration date of April 2017. But the last lot manufactured by USPlabs, according to the company, had an expiration date of October 2016. "The counterfeit product expiration date of 04/17 is therefore obviously fraudulent," the statement said.
Fluoxetine is a selective serotonin reuptake inhibitor, or SSRI, a class of prescription drug that is commonly used to treat depression, obsessive-compulsive disorder, and panic disorder.
"Uses of SSRIs have been associated with serious side effects including suicidal thinking, abnormal bleeding, and seizures," the FDA letter said.
USPlabs has a history of FDA sanctions. In 2011, an earlier version of the product, called OxyElite Pro, was found to contain dimethylamylamine, or DMAA, a synthetic stimulant linked to bleeding strokes and sudden deaths. The FDA banned DMAA in 2012, but USPlabs kept selling the stimulant for more than a year.
After the company removed DMAA, it reformulated OxyElite Pro and added a new ingredient, called aegeline. In May 2013, this new version of the supplement spurred an outbreak of liver disease, and six months later the company recalled it. A subsequent investigation by the Centers for Disease Control and Prevention linked OxyElite Pro to 97 cases of hepatitis, resulting in 47 hospitalizations, three liver transplantations, and one death.
"The current system to eliminate dangerous supplements in the U.S. is fatally flawed," Pieter Cohen, assistant professor of medicine at Harvard Medical School, told BuzzFeed News. "Loosey-goosey laws combined with the FDA's lackadaisical approach to regulating the supplement industry places millions of consumers at risk."
OxyElite Pro can be sold without going through a formal FDA-approval process because it is considered a dietary supplement. "These products are typically promoted for sexual enhancement, weight loss, and body building and are often represented as being 'all natural,'" the FDA letter read.
But consumers should be warned that many supplements — at least 574 products identified so far — are tainted with synthetic pharmaceutical compounds.
"FDA is unable to test and identify all products marketed as dietary supplements that have potentially harmful hidden ingredients," the letter said.
This post has been updated to include USPlabs' claim that the tainted supplement is a counterfeit.
Virginia Hughes is the science editor for BuzzFeed News and is based in New York.
Contact Virginia Hughes at firstname.lastname@example.org.
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